This invention relates to an electrostatographic printing machine, and more particularly concerns an improved transfer system for use therein.
In electrostatographic printing, an electrostatic latent charge pattern is created and reproduced in viewable form. The field of electrostatography includes electrophotography and electrography. Electrophotography employs a photosensitive medium to form, with the aid of electromagnetic radiation, an electrostatic latent charge pattern. Contrawise, electrography utilizes an insulating medium to form, without the aid of electromagnetic radiation, the electrostatic latent charge pattern. The process of transferring toner particles deposited on the electrostatic latent charge pattern, in image configuration, to a sheet of support material, is employed in both of the preceding types of electrostatographic printing.
Hereinafter, a color electrophotographic printing machine will be described as an illustrative embodiment of the foregoing process wherein the transfer apparatus of the present invention may be employed. In the process of electrophotographic printing, for example as disclosed in U.S. Pat. No. 2,297,691 issued to Carlson in 1942, an image bearing member or photosensitive element having a photoconductive insulating layer is charged to a substantially uniform potential in order to sensitize the surface thereof. Thereafter, the charged photoconductive surface is exposed to a light image of an original document. As a consequence of the exposure, the charge is selectively dissipated in the irradiated areas in accordance with the light intensity projected onto the charged photoconductive surface. This records an electrostatic latent charge pattern or an electrostatic latent image corresponding to the original document. Development of the electrostatic latent image is achieved by bringing a developer mix into contact therewith. Typical developer mixes employ colored thermoplastic particles, i.e., toner particles, which are mixed with ferromagnetic granules, i.e., carrier granules. The developer mix is selected such that the toner particles acquire the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. As the developer mix is moved into contact with the photoconductive surface, the greater attractive force of the electrostatic latent image recorded thereon causes the toner particles to be separated from the carrier granules and adhere to the electrostatic latent image. Thereafter, the toner powder image adhering to the electrostatic latent image is transferred to a sheet of support material such as a sheet of paper, or a thermoplastic sheet amongst others. The toner powder image is then permanently affixed thereto.
One type of transfer apparatus is described in U.S. Pat. No. 3,357,325 issued to Eichorn et al. in 1967. As disclosed therein, a transfer corona generating device is disposed beneath an endless belt arranged to move a sheet of support material into contact with the photoconductive drum. The transfer corona generating device applies a charge to the back of the belt. The electrostatic charge placed on the back of the belt by the corona generating device attracts the powder image from the drum surface to the sheet of support material.
With the advent of multi-color electrophotographic printing, successive layers of toner powder images are transferred to the sheet of support material in superimposed registration with one another. In multi-color electrophotographic printing, successive single color electrostatic latent images are created on the photoconductive surface and developed with correspondingly colored toner particles. Thereafter, each single color toner powder image is transferred to the sheet of support material in superimposed registration with the prior one. Thus, it is evident that the sheet of support material moves in a recirculating path to receive successive toner powder images in superimposed registration with one another. This may be achieved by the employment of a transfer roll. The transfer roll is electrically biased to generate a high voltage discharge in the proximity of the surface of the sheet of support material or it may be applied by means of a conductive cylinder in contact with the paper as is disclosed in U.S. Pat. No. 2,807,233 issued to Fitch in 1957. As described therein, a sheet of support material is interposed between the conductive roller and a surface having the toner powder image thereon. A charge of opposite polarity from the toner particles is deposited on the back side of the sheet of support material which attracts the toner powder image thereto. U.S. Pat. No. 3,612,677 issued to Langdon et al. in 1971 also describes an electrically biased transfer roll. This system is particularly adapted for multi-color electrophotographic printing. As disclosed therein, a transfer roll moves the sheet of support material in a recirculating path. The transfer roll is biased electrically to a potential of sufficient magnitude and polarity to attract electrostatically toner particles from the electrostatic latent image recorded on the photoconductive surface to the sheet of support material. The transfer roll rotates in synchronism with the photoconductive drum. Inasmuch as the sheet of support material is secured releasably on the transfer roll for movement in a recirculating path therewith, successive toner powder images may be transferred thereto in superimposed registration with one another. It should be noted that a transfer roll requires some mechanism for securing the sheet of support material to the transfer roll. This is frequently accomplished by employing gripper fingers arranged to grasp the leading edge of the sheet of support material, thereby securing the sheet of support material to the transfer roll. However, an arrangement of this type may cause edge deletion, i.e., copying without a bleed. Moreover, the foregoing type of system is relatively complex and expensive to manufacture. In addition, it requires a change in the paper feed path. Generally, the paper feed path employs a plurality of endless conveyor belts adapted to advance the sheet of support material from the sheet tray to the catch tray for subsequent removal therefrom by the operator. The conveyors transport the sheet of support material sequentially through the respective processing station to produce the desired copy thereon. However, with the employment of a transfer roll, this simple feed path is interrupted and additional complexities are added to the system. Thus, it would be highly desirable to employ a flat surface or endless conveyor belt in lieu of the transfer roll when creating multi-color copies.
Accordingly, it is a primary object of the present invention to improve the apparatus for transferring toner powder images from a photoconductive surface to a sheet of support material.